Photoconductive target of an image pickup tube comprising graded selenium-tellurium layer

ABSTRACT

A photoconductive target of an image pickup tube comprising a light-transmitting substrate, an N-type conductive layer deposited on the substrate and a P-type conductive layer making a rectifying contact with the N-type conductive layer, in which the P-type conductive layer includes at least selenium and tellurium, the composition of the P-type layer changes along the direction of the thickness of the layer, the average content of selenium in the P-type conductive layer is not less than 50 atomic percent, the content of tellurium at both surfaces of the P-type conductive layer is not more than 10 atomic percent, and the maximum tellurium content of 10 to 40 atomic percent is located on a plane in the P-type conductive layer nearer to the N-type conductive layer than the middle plane of the P-type conductive layer.

United States Patent Hirai et al.

[ PHOTOCONDUCTIVE TARGET OF AN IMAGE PICKUP TUBE COMPRISING GRADEDSELENIUM-TELLURIUM LAYER [75] Inventors: Tadaaki Hirai, Koganei; EiichiMaruyama, Kodaira; Kiyohisa lnao, Hachioji; Hideaki Yamamoto, Kokubunji;Naohiro Goto; Yukinao lsozaki, both of Machida; Keiichi Shidara; TeruoUchida, both of Tokyo. all of Japan [73] Assignees: Hitachi, Ltd.;Nippon Hoso Kyol-tai,

both of Japan [22] Filed: June 15, 1973 [21] Appl. No.: 370,446

[52] U.S. Cl 313/386; 313/94 [51] Int. Cl l-lOij 29/45; H0lj 31/38 [58]Field of Search 3l3/386, 385, 384, 65 A [56] References Cited UNITEDSTATES PATENTS 3,346,755 l0/l967 Dresner 3l3/65 A June 17, 19753,350,595 lO/l967 Kramer 313/65 A X Primary Examiner--Robert SegalAttorney, Agent, or Firm-Craig & Antonelli [57] ABSTRACT Aphotoconductive target of an image pickup tube comprising alight-transmitting substrate, an N-type conductive layer deposited onthe substrate and a P- type conductive layer making a rectifying contactwith the N-type conductive layer, in which the P-type conductive layerincludes at least selenium and tellurium, the composition of the P-typelayer changes along the direction of the thickness of the layer, theaverage content of selenium in the P-type conductive layer is not lessthan 50 atomic percent, the content of tellurium at both surfaces of theP-type conductive layer is not more than l0 atomic percent, and themaximum tellurium content of 10 to 40 atomic percent is located on aplane in the P-type conductive layer nearer to the N-type conductivelayer than the middle plane of the P-type conductive layer.

10 Claims, 5 Drawing Figures PATENTEDJUII I 7 I975 SlIEEI FIG. 5

FIG.

3 220.53 ZOEmOnSOO I/ 1/ |\l L /1 FIG. 2

3 220:: ZOEmOmZOU THICKNESS 0F FILM (F PATEN'IEIIJIIII I 7 I975COMPOSITION (ATOMIC 95) FIG. 3

g 20 As Tmcmess 0F FILM u FIG. 4

1 PHOTOCONDUCTIVE TARGET OF AN IMAGE PICKUP TUBE COMPRISING GRADEDSELENIUM-TELLURIUM LAYER The present invention relates generally to theconstruction of a photoconductive layer used for the target of an imagepickup tube of the vidicon type, or more particularly to aphotoconductive layer with a rectifying contact which has an increasedsensitivity and an improved spectral sensitivity to red light.

Sb S PbO and Si are widely used as materials of photoconductive layersfor the target of the vidicon type image pickup tube. Among thesematerials, Sb S constitutes a photoconductive layer of injecting contacttype, while PhD and Si are used for the photoconductive layers ofrectifying contact or junction type. The advantages of thephotoconductive layer of rectifying contact or junction type over thatof injection type are a higher response speed, small dark current andhigher sensitivity. However, the photoconductive materials capable offorming a rectifying contact successfully used as a target of the imagepickup tube are limited, and it is difficult to obtain a photoconductivematerial with properties suitable in all respects.

The peak of spectral sensitivity, for example, is located in theproximity of infrared range for Si and on the side of visible shortwavelength for PhD, with the result that if they are used for a colorimage pickup tube, Si and PbO have insufficient sensitivity to blue andred respectively. The inventors have found that amorphous selenium isalso capable of forming a rectifying contact suitable for the target ofthe image pickup tube, but this material also has the disadvantage ofinsufficient sensitivity to red light.

A method to improve the sensitivity to red light is disclosed in U.S.Pat, No. 3,350,595. According to this method, a conductive thin film isdeposited on an insulating substrate which is in turn covered with aphotoconductive layer comprising mainly a mixture of tellurium andselenium. The surface portion of the photoconductive layer adjacent tothe conductive thin film contains selenium of 70 to 80 by weight whilethe opposite surface portion thereof includes selenium of 90 to I byweight, the selenium content gently changing between both the surfaceportions. However, the fact that the tellurium content in thephotoconductive layer near the interface thereof with the conductivethin film is high is accompanied by the disadvantage of increased darkcurrent. In order to overcome this disadvantage, U.S. Pat. No. 3,350,595discloses a method in which a blocking layer of such metal as cesiumwith small work function is interposed between the conductive film andthe photoconductive layer. This method, however, is disadvantageous inthat the manufacturing processes are complicated.

The object of the present invention is to obviate the above-mentioneddisadvantages and to provide a photoconductive target of an image pickuptube which is high in sensitivity to red light, small in dark currentand is easily manufactured.

In order to achieve the above-mentioned object, the photoconductivetarget of the image pickup tube according to the invention comprises alight-transmitting substrate, a first N-type conductive layer depositedon said substrate and a P-type conductive layer making a rectifyingcontact with said first N-type conductive layer, said P-type conductivelayer including at least selenium and tellurium, the composition of saidP-type conductive layer being different along the direction of thethickness thereof, the average content of selenium in said P-typeconductive layer being not less than 50 atomic percent, the content oftellurium at both surfaces of said P-type conductive layer being notmore than l0 atomic percent, the maximum tellurium content of 10 to 40atomic percent being located on a plane in said P-type conductive layernearer to said first N-type conductive layer than the middle plane ofsaid P-type conductive layer. The N-type conductive layer is alight-transmitting conductive film preferably including as its maincomponent an oxide of tin, indium or titanium. Further, to prevent thecrystallization of the P-type conductive layer, an N-type conductivelayer formed of one substance selected from the group consisting of CdS,CdSe, ZnS, ZnSe and a mixture thereof may be interposed between theP-type conductive layer and the light-transmitting conductive film or atranslucent metal making up the surface portion of thelight-transmitting substrate nearer to the side of the N-type conductivefilm. The P-type conductive layer may include As, Sb, P, Bi, Ge and/orSi in addition to selenium and tellurium.

Amorphous selenium is generally of P conduction type and forms arectifying contact with a variety of N- type materials including filmsof single crystals or crystallites of such lV group semiconductors as Geand Si, [I] V group semiconductors such as GaAs and GaP and II VI groupsemiconductors. Among them, the most suitable material of N-typeconduction to form a photoconductive layer for the target of an imagepickup tube in combination with selenium are an oxide of tin used for atransparent conductive film, an oxide of indium, an oxide of titanium,and II Vl group semiconductors such as ZnS, ZnSe, CdS and CdSe. Althoughthe low intrinsic resistance of conductive films of the above-mentionedoxides makes it possible to use them also as an electrode for taking outa signal from the image pickup tube, the II Vl group semiconductorscannot be used as such an electrode at the same time without additionalprovision of the transparent electrode of any of the above-mentionedoxides or a translucent metal laid thereon.

The better understanding of the present invention will be gained fromthe following description taken in conjunction with the accompanyingdrawings in which:

FIG. I is a sectional view showing the fundamental construction of thetarget of the image pickup tube according to the present invention; and

FIGS. 2 to 5 are graphs illustrating the distribution of componentelements along the direction of the thickness of the P-typephotoconductive layer.

An embodiment of the invention is shown in FlG. I. The target of animage pickup tube according to the invention generally comprises a glasssubstrate 1, a transparent electrode 2 extended on the glass substrate1, an N-type conductive layer 3 and a P-type conductive layer 4.Reference numeral 5 shows incident light, and numeral 6 a scanningelectron beam. A rectifying contact is formed between the N-typeconductive layer 3 and the P-type conductive layer 4.

[n the case where the transparent electrode 2 is formed of an oxide ofN-type conductivity, the transparent electrode 2 and the N-typeconductive electrode 3 are actually integrated into a single layer. Inthe event that amorphous selenium and the above-mentioned oxide or a llVI group semiconductor are used as the materials of the P-typeconductive layer and the N-type conductive layer respectively,sensitization is necessary to improve the sensitivity to red light sincethe abovementioned materials other than CdSe do not have sen sitivity tored light.

A well-known method to improve sensitivity of ll Vl group to red lightconsists in doping into it Cl, Br, I, In, Ga or other elements acting asa donor together with Cu, Ag or the like element forming an acceptor. Inthe case where a P-type conductive layer including selenium is involvedas in the present invention, the sensitivity to red light issuccessfully improved in combination with the above-mentioned methodconcerning the sensitization of the N-type conductive layer of ll Vlgroup semiconductor.

It is also well known that the sensitivity to red light can be improvedby adding Te to amorphous selenium. The electrical resistance of Se towhich Te has been added is sharply reduced. thereby degrading the char--acteristics ofthe target of the image pickup tube. If, for example, theconcentration of Te in the neighborhood ofthe interface between N-typeconductive layer 3 and P-type conductive layer 4 is increased, thereverse breakdown voltage of the rectifying contact is decreased therebyto increase the dark current in the image pickup tube.

On the other hand, if the Te concentration of the P- type conductivelayer 4 as a whole is increased, the resistance of the P-type layer isdecreased or the carrier mobility in the layer is reduced, with theresult that the dark current is increased or the time responsecharacteristics are degraded.

The results of research by the inventors show that in improving thesensitivity of the Se conductive layer to red light by the use of Te, itis recommended that Te concentration in the Sc conductive layer be madepro gressively higher toward the side of the N-type conductive layerrather than uniformly distributing it, and the Te concentration at andin the vicinity of the interface between N-type conductive layer andP-type conductive layer is maintained at a low level, thus making itpossible to prevent dark current from being increased without adverselyaffecting the characteristics of the target of the image pickup tube.For this purpose, it is necessary to limit the Te concentration at andin the vi cinity of the interface to a level ranging from to atomicpercent. Also, if there is Te included in a considerable portion of theP-type conductive layer toward the opposite surface thereof, theintensity of electric field in such a portion is decreased for thedegradation of the response characteristic thereof, and therefore it isnecessary to maintain the Te concentration in that portion in the rangefrom O to 10 atomic percent.

The Te concentration of as little as l0 atomic percent in the P-typeconductive layer cannot achieve sufficient improvement in sensitivity tored light. In view of this, the most effective method to improve thesensitivity to red light is to provide a portion between the surfaces ofthe P-type conductive layer with the main component of Se where the Teconcentration is maximum. If the excitation of carriers is to beeffected satisfactorily at the point of high Te concentration, it isdesirable that such a portion be located at a position the nearestpossible to the plane of incidence ofa light signal into the P-typeconductive layer, that is, the interface thereof with the N-typeconductive layer. In other words, the

portion of maximum Te concentration should be located on a plane in theP-type conductive layer nearer to the N-type conductive layer than themiddle plane of the P-type conductive layer.

Since an excessively high Te concentration in that portion results in anincreased dark current, the maximum value of Te concentration shouldpreferably be between [0 to 40 atomic percent. As will be explainedlater with reference to an embodiment, Te is not necessarily required tobe smoothly distributed in the P-type conductive layer, but it mayconsist of a laminated structure of a multiplicity of films each about10 A thick comprising films of high Te concentration and films of low Teconcentration laid one on another. For this reason, it should be notedthat the abovementioned range of Te concentration from 10 to 40 atomicpercent represents an average value in the region several hundred Athick including the portion of maximum Te concentration.

The progressive change in Te concentration in the transverse directionof the P-type conductive layer may be abrupt in the microscopic orderof, say, several tens of A. Somewhat macroscopically, however, the curveof the change should preferably be gentle in the order of severalhundreds of A. If macroscopically there is a portion where the Teconcentration is discontinued, the burning effect of the image pickuptube may be promoted.

The disadvantage of the photoconductive layer with Se as a maincomponent resides in that the layer is easily crystallized by heat, withthe result that the picture produced is accompanied by defects in theform of white dots. By a well-known method to prevent the defects, anelement such as As, Sb, P, Bi, Ge or Si is added to the material for thelayer thereby to increase the viscosity thereof and delay the speed ofcrystallization. This principle also applies to the present invention,wherein the life of the target may be lengthened by adding such anelement to the P-type conductive layer thereby to reduce the speed ofcrystallization. The addition of excessive amount of the elementadversely affects the response characteristic of the target, thedesirable amount of the element to be added being less than 20 atomicpercent.

When the element for prevention of the crystallization coexists withtellurium for improving the sensitivity, the maintaining of superiordark current and response characteristics require that selenium accountsfor at least 50 atomic percent.

The fact that the emission of secondary electrons from thephotoconductive layer containing much Se used as a target iscomparatively great disturbs the landing of the scanning beam and oftencauses abnormal phenomena including the distortion of an image and thereversal ofa polarity ofa video signal at a high target voltage.

An effective method to prevent the above-mentioned phenomena is todeposit by vacuum or gaseous evaporation on the P-type conductive layera film of Sb S As Se or A5 8 approximately 1000 A thick.

Embodiments of the invention will be explained below.

Embodiment 1 Se, Ge and Te contained in different. evaporation boats oftantalum are deposited simultaneously by evaporation in the vacuum of 3X It) Torr on a transparent N-type conductive layer with tin oxide as amain component which is formed on a glass substrate. In this way, aP-type photoconductive layer as thick as 3 pm is produced.

The compositional profile of the P-type photoconductive layer isadjusted by controlling the current in each boat so as to be inconsistence with the graph as shown in FIG. 2. Further, an Sb S filmapproximately 1000 A thick is deposited by evaporation on the sur faceof the P-type photoconductive layer in the lowpressure argon of 5 X 10Torr thereby to improve the landing characteristic of the scanning beamfor the target of an image pickup tube.

Embodiment 2 A transparent N-type layer consisting mainly of indiumoxide is deposited on a glass substrate. A CdSe film 2000 A thick isfurther deposited in the vacuum of 2 X 10 Torr at the substratetemperature of 200C, on the surface ofa transparent N-type conductivelayer by evaporation. On the other hand, a first photoconductivematerial of Se containing Te of 40 atomic percent and a secondphotoconductive material of Se containing As of IO atomic percent areprepared in a quartz ampule. These two types of photoconductivematerials are crushed and put into different evaporation boats oftantalum and then they are simultaneously deposited on the CdSe layer inthe vacuum of 3 X lo Torr. In the process, the speed of evaporation ofthe first and second photoconductive materials is continuously changedto form a film 4 pm thick with the composition distribution of Se, Teand As as shown in the graph of FIG. 3. On the surface of the resultingP-type conductive layer is deposited by evaporation a film of A5 Seapproximately 500 A thick in the vacuum of 3 X 10 Torr. This As Se filmis further covered by a similar method with an As Se film about 500 Athick in the low-pressure argon of 5 X l Torr. This double layer of AsSe is formed for the purpose of improving the landing characteristic ofthe scanning electron beam.

Embodiment 3 A translucent A] film is formed on a glsss substrate in thevacuum of l X l0 Torr at the substrate temperature of 150C, and then onthis translucent Al film is deposited a CdS film 3000 A thick in thevacuum of X 10 Torr at the substrate temperature of l50C.

Subsequently, thin films of Se, As Se and Te are deposited one afteranother on the CdS film in a rotary vacuum evaporator of 5 X 10" Torr.As a result, a film 5 pm thick comprising 3000 to 6000 layers of Se, AsS63 and Te each having the average thickness of 10 A or less isobtained.

During the process of forming the multiple layer, the current in the Teboat or the opening of a slit interposed between the Te evaporation boatand the substrate is controlled continuously thereby to produce amacroscopic profile of transverse distribution of composition as shownin H0. 4. On this multiple layer is further deposited an A5 5 film about500 A thick in the vacuum of 2 X l0 Torr thereby to improve the landingcharacteristic of the scanning electron beam.

Embodiment 4 Films of CdSe, CdTe and Se are deposited one after anotherin the vacuum of 5 X 10 Torr on a transparent N-type conductiveelectrode with tin oxide as a main component which is in turn depositedon a glass substrate. In this way, a multiple layer 2 pm thickcomprising 2000 to 4000 films of CdSe, CdTe and Se each having theaverage thickness of 10 A or less is obtained. ln the process offormingthe multiple layer. the macroscopic composition of the elements in thetransverse direction of the layer is controlled by similar means tothose employed in the preceding embodiment thereby to obtain the profileof composition as shown in FIG. 5.

An excessive amount of Cd tends to be included in the CdSe film by theordinary method of production thereof, often resulting in the CdSe filmhaving an N- type of conduction. This problem is overcome by adding Seas in the embodiment under consideration thereby to obtain an intrinsicor nearly P-type layer. On this layer is further deposited an Sb- S filmapproximately 1000 A thick in the low-pressure argon of 5 X 10 Torr forimproving the landing characteristic of the scanning beam.

it will be understood from the above explanation of the embodiments thataccording to the invention the sensitivity especially to red light isimproved without adverse effect on the rectifying contact with theN-type photoconductive layer by distributing Te in a P-typephotoconductive layer containing Sc or 50 or more atomic percent in sucha manner that the maximum Te content is located on a plane in the P-typeconductive layer nearer to the N-type conductive layer than the middleplane of the Ptype conductive layer. The spectral sensitivity of theP-type conductive layer is thus changed greatly, making it possible toproduce a photoconductive layer suitable for a specific purpose. Although the above explanation of the embodiments in volves alight-receiving film of the target for the image pickup tube, it ispossible to use the above-mentioned photoconductive layer for asolid-state light-receiving element, solid-state image pickup element orthe like by employing an appropriate metal electrode in place of anelectron beam.

What we claim is:

l. A photoconductive target of an image pickup tube comprising alight-transmitting substrate, a first N-type conductive layer depositedon said substrate and a P- type conductive layer making a rectifyingcontact at a first surface thereof with said first N-type conductivelayer and having a second outer surface to be scanned by electrons, saidP-type conductive layer including at least selenium and tellurium, thecomposition of said P-type conductive layer being different along thedirection of the thickness thereof, the average content of selenium insaid P-type conductive layer being not less than 50 atomic percent, thecontent of tellurium at said first and second surfaces of said P-typeconductive layer being not more than 10 atomic percent, the maximumtellurium content of 10 to 40 atomic percent being located on a plane insaid P-type conductive layer between said first N-type conductive layerand the middle plane of said P-type conductive layer.

2. A photoconductive target of an image pickup tube according to claim1, wherein said first N-type conduc tive layer is a transparentconductive film including one substance selected from the groupconsisting of an oxide of tin. indium, and titanium as a main component.

3. A photoconductive target of an image pickup tube according to claimI, wherein a second N-type conductive layer formed of one substanceselected from the group consisting of CdS, CdSe. ZnS, ZnSe and themixture thereof is interposed between said P-type conductive layer andsaid first N-type conductive layer or a translucent metal electrodeconstituting the surface portion of said lighttransmitting substrate onthe side of said first N-type conductive layer.

4. A photoconductive target of an image pickup tube according to claimI, wherein said P-type conductive layer contains one substance selectedfrom the group consisting of As, Sb. P. Bi, Ge, Si and the mixturethereof in addition to selenium and tellurium.

5. A photoconductive target of an image pickup tube according to claim4, wherein the concentration of said one substance in said P-typeconductive layer is substantially uniform therethrough.

6. A photoconductive target of an image pickup tube according to claim1, wherein the minimum concentra tion of said selenium in said P-typeconductive layer is located at the location of said maximum telluriumcontent in said P-type conductive layer.

7. A photoconductive target of an image pickup tube according to claimI, wherein said P-type conductive layer contains Cd in addition toselenium and tellurium.

8. A photoconductive target of an image pickup tube according to claim7, wherein the maximum concentration of said Cd in said P-typeconductive layer is located at the location of said maximum telluriumcontent in said P-type conductive layer.

9. A photoconductive target of an image pickup tube according to claim1, wherein the absolute rate of increase of the tellurium content insaid P-type conductive layer as the location of said maximum telluriumcontent is approached is equal to the absolute rate of decrease of thetellurium content as the location of said maximum tellurium content ispassed.

10. A photoconductive target of an image pickup tube according to claim1, wherein the absolute rate of increase of the tellurium content insaid P-type conductive layer as the location of said maximum telluriumcontent is approached is greater than the absolute rate of decrease ofthe tellurium content as the location of said maximum tellurium contentis passed.

1. A PHOTOCONDUCTIVE TARGET OF AN IMAGE PICKUP TUBE COMPRISING ALIGHT-TRANSMITTING SUBSTRATE, A FIRST N-TYPE CONDUCTIVE LAYER DEPOSITEDON SAID SUBSTRATE AND A P-TYPE CONDUCTIVE LAYER MAKING A REACTIFYINGCONTACT AT A FIRST SURFACE THEREOF WITH SAID FIRST N-TYPE CONDUCTIVELAYER AND HAVING A SECOND OUTER SURFACE TO BE SCANNED BY ELECTRONS, SAIDP-TYPE CONDUCTIVE LAYER INCLUDING AT LEAST SELENIUM AND TELLURIUM, THECOMPOSITION OF SAID P-TYPE CONDUCTIVE LAYER BEING DIFFERENT ALONG THEDIRECTION OF THE THICKNESS THEREOF, THE AVERAGE CONTENT OF SELENIUM INSAID P-TYPE CONDUCTIVE LAYER BEING NOT LESS THAN 50 ATOMIC PERCENT, THECONTENT OF TELLURIUM AT SAID FIRST AND SECOND SURFACES OF SAID P-TYPECONDUCTIVE LAYER BEING NOT MORE THAN 10 ATOMIC PERCENT, THE MAXIMUMTELLURIUM CONTENT OF 10 TO 40 ATOMIC PERCENT BEING LOCATED ON A PLANE INSAID P-TYPE
 2. A photoconductive target of an image pickup tubeaccording to claim 1, wherein said first N-type conductive layer is atransparent conductive film including one substance selected from thegroup consisting of an oxide of tin, indium, and titanium as a maincomponent.
 3. A photoconductive target of an image pickup tube accordingto claim 1, wherein a second N-type conductive layer formed of onesubstance selected from the group consisting of CdS, CdSe, ZnS, ZnSe andthe mixture thereof is interposed between said P-type conductive layerand said first N-type conductive layer or a translucent metal electrodeconstituting the surface portion of said lighttransmitting substrate onthe side of said first N-type conductive layer.
 4. A photoconductivetarget of an image pickup tube according to claim 1, wherein said P-typeconductive layer contains one substance selected from the groupconsisting of As, Sb, P, Bi, Ge, Si and the mixture thereof in additionto selenium and tellurium.
 5. A photoconductive target of an imagepickup tube according to claim 4, wherein the concentration of said onesubstance in said P-type conductive layer is substantially uniformtherethrough.
 6. A photoconductive target of an image pickup tubeaccordIng to claim 1, wherein the minimum concentration of said seleniumin said P-type conductive layer is located at the location of saidmaximum tellurium content in said P-type conductive layer.
 7. Aphotoconductive target of an image pickup tube according to claim 1,wherein said P-type conductive layer contains Cd in addition to seleniumand tellurium.
 8. A photoconductive target of an image pickup tubeaccording to claim 7, wherein the maximum concentration of said Cd insaid P-type conductive layer is located at the location of said maximumtellurium content in said P-type conductive layer.
 9. A photoconductivetarget of an image pickup tube according to claim 1, wherein theabsolute rate of increase of the tellurium content in said P-typeconductive layer as the location of said maximum tellurium content isapproached is equal to the absolute rate of decrease of the telluriumcontent as the location of said maximum tellurium content is passed. 10.A photoconductive target of an image pickup tube according to claim 1,wherein the absolute rate of increase of the tellurium content in saidP-type conductive layer as the location of said maximum telluriumcontent is approached is greater than the absolute rate of decrease ofthe tellurium content as the location of said maximum tellurium contentis passed.